The definition of an oncogene is a gene that actively promotes tumorigenesis. Paradoxically, activation of oncogenes, such as RAS, in primary mammalian cells usually triggers cellular senescence, which causes permanent growth arrest and thus provides a mechanism to suppress tumorigenesis. Our broad long-term goal is to understand the molecular basis underlying this paradox. Senescent cells are typically characterized by the presence of DNA damage. One possible reason for the linkage of RAS-dependent transformation and senescence might be that damage creates a mutagenic environment that supports transformation before establishing senescence. BRCA1 plays a critical role in the repair of DNA damage and its inactivation creates a mutagenic environment that drives transformation. The rationale for the proposed research is that BRCA1 is released from chromatin prior to senescence-induced cell cycle exit. Repression of BRIP1, a BRCA1 physiological binding partner in the DNA repair pathway, at transcriptional level triggers BRCA1 chromatin dissociation. BRCA1 release is essential for senescence, as ectopic BRIP1 prevents BRCA1 chromatin dissociation and blocks senescence. BRCA1 chromatin dissociation may also influence senescence through regulation of chromatin structure. For example, BRCA1 interacts with BRG1, an ATP dependent chromatin remodeler. Our preliminary studies show that the interaction between BRCA1 and BRG1 is disrupted during senescence and this coincides with an increased level of chromatin-associated BRG1. Mimicking this effect by knocking down BRCA1 or ectopically expressing BRG1 triggers senescence. The objective of this proposal is to determine the role of the BRCA1 pathway in regulation of oncogene-induced senescence. Our central hypothesis is that oncogene-induced BRCA1 chromatin dissociation in primary cells promotes senescence in the vast majority of cells. Further, it creates a mutagenic environment by inactivating BRCA1-mediated DNA repair, which allows for acquisition of secondary hits and ultimately diverts the fate of a minority of cells to senescence bypass and transformation. This hypothesis will be tested in the following three specific aims: 1) To investigate the mechanisms underlying BRIP1 repression during RAS-induced senescence; 2) To investigate the mechanism by which an altered interaction between BRG1 and BRCA1 promotes senescence; and 3) To investigate BRCA1-mediated DNA repair response in cells undergoing senescence, and determine whether DNA damage promotes senescence bypass and transformation. The research proposed is innovative because it focuses on a novel cell-intrinsic mechanism that promotes senescence but at the same time predisposes cells to transformation. The research proposed is significant because it will reconcile a major paradox in the oncogene-induced senescence field. The molecular insights gained from our studies will reveal how activated oncogenes promote senescence while predisposing cells to transformation with broad implications for cancer biology.